1. Field of the Invention
The present invention relates to methods and apparatus for laser wavefront diagnostics and, in particular, to the method and apparatus for measurement of the parallelism of the rays of light within a laser beam. Other geometric parameters such as beam pointing and centering, may also be measured using these diffractive optical elements.
2. Description of the Related Art
The quality of a laser beam can be defined by a number of factors. One such factor is the localized slope error of the beam. The localized slope error, which is typically measured utilizing a wavefront diagnostic apparatus, defines the parallelism between different bundles of light rays within a laser beam.
The operation of a wavefront diagnostic apparatus can best be understood by first considering the operation of a pointing diagnostic apparatus. A simple pointing diagnostic apparatus, which measures the incident angle formed between an incoming laser beam and the surface of an object struck by the incoming laser beam, can be formed by placing a viewing screen in back of an opaque object that has a pinhole aperture formed through the surface of the opaque object. When the opaque object is illuminated by the laser beam, the pinhole aperture forms a spot of light on the viewing screen. The position of the spot of light on the screen is related to the incident angle of the incoming beam. If a reference beam also illuminated the opaque object, the difference in position between the two spots would be related to the angular difference between the incoming beams.
The measurement of the parallelism of rays within a beam may be determined using an array of pointing diagnostics. A "Hartmann diagnostic apparatus" is a wavefront diagnostic device consisting of an array of holes in a perforated plate. As with the pointing diagnostic apparatus, by comparing the position of each spot of the array of spots produced by a collimated beam of light to the position of a corresponding spot produced by the test beam of light, where both the collimated beam and the test beam emanate from the same location, the incident angle of each bundle of light rays can be determined.
An enhanced version of the Hartmann diagnostic apparatus, known as the Hartmann-Shack diagnostic apparatus, employs an array of converging lenslets in place of the array of pinhole apertures. The array of converging lenslets captures the light that strikes each lenslet and focuses the light onto the viewing screen as an array of focused spots.
The array of focused spots has the advantage of simplifying the task of comparing the relative positions of each image because it is easier to determine the positional variation of a focused spot than it is to determine the positional variation of fuzzy blobs of light.
In addition to the array of converging lenslets, the Hartmann-Shack diagnostic apparatus may employ the detection array of a video camera as the viewing screen. The utilization of a video detection system provides a means for integrating computerized positional processing into the Hartmann-Shack diagnostic apparatus, thereby enhancing the speed and accuracy of a parallelism measurement.
Although the Hartmann-Shack diagnostic apparatus has several advantages, there are several disadvantages as well. First, by utilizing the detection array of a video camera, which is typically 6.6 by 8.8 millimeters in size, the diameter of a laser beam, which can range in size from one millimeter to a meter in diameter, must typically be altered to match the diameter of the detector. The process of increasing or reducing the diameter of a laser beam requires the precise positioning of a number of mirrors and/or lenses, each of which introduce aberrations into the resulting laser beam and increase the cost and complexity of the parallelism measurement.
Second, since the array of converging lenslets captures almost all of the light which strikes the lenslets, a dedicated laser beam must be used to make the parallelism measurements. This requires that a primary laser beam be split to produce the dedicated laser beam, further increasing the cost and complexity of the parallelism measurement.
Third, a laser beam is frequently comprised of multiple colors or frequencies. When multiple frequencies are present, it is typically desirable to test the parallelism of each frequency. In the Hartmann-Shack diagnostic apparatus, multiple color testing requires the repeated utilization of cumbersome color filters to remove the unwanted frequencies.
Thus, there is a need for a wavefront diagnostic apparatus which does not require beam diameter sizing, the separate formation of a dedicated test beam, and the use of color filters.